[PATCH v4 3/5] plugins/cache: Added FIFO and LRU eviction policies

Mahmoud Mandour posted 5 patches 3 months ago
Maintainers: "Alex Bennée" <alex.bennee@linaro.org>

[PATCH v4 3/5] plugins/cache: Added FIFO and LRU eviction policies

Posted by Mahmoud Mandour 3 months ago
Implemented FIFO and LRU eviction policies. Now one of the three
eviction policies can be chosen as an argument. On not specifying an
argument, LRU is used by default.

Signed-off-by: Mahmoud Mandour <ma.mandourr@gmail.com>
---
 contrib/plugins/cache.c | 203 +++++++++++++++++++++++++++++++++++++---
 1 file changed, 190 insertions(+), 13 deletions(-)

diff --git a/contrib/plugins/cache.c b/contrib/plugins/cache.c
index b550ef31b0..bf0d2f6097 100644
--- a/contrib/plugins/cache.c
+++ b/contrib/plugins/cache.c
@@ -29,6 +29,14 @@ static uint64_t dmisses;
 static uint64_t imem_accesses;
 static uint64_t imisses;
 
+enum EvictionPolicy {
+    LRU,
+    FIFO,
+    RAND,
+};
+
+enum EvictionPolicy policy;
+
 /*
  * A CacheSet is a set of cache blocks. A memory block that maps to a set can be
  * put in any of the blocks inside the set. The number of block per set is
@@ -48,6 +56,8 @@ static uint64_t imisses;
  * The set number is used to identify the set in which the block may exist.
  * The tag is compared against all the tags of a set to search for a match. If a
  * match is found, then the access is a hit.
+ *
+ * The CacheSet also contains bookkeaping information about eviction details.
  */
 
 typedef struct {
@@ -57,6 +67,9 @@ typedef struct {
 
 typedef struct {
     CacheBlock *blocks;
+    uint64_t *lru_priorities;
+    uint64_t lru_gen_counter;
+    GQueue *fifo_queue;
 } CacheSet;
 
 typedef struct {
@@ -77,6 +90,12 @@ typedef struct {
     uint64_t imisses;
 } InsnData;
 
+void (*update_hit)(Cache *cache, int set, int blk);
+void (*update_miss)(Cache *cache, int set, int blk);
+
+void (*metadata_init)(Cache *cache);
+void (*metadata_destroy)(Cache *cache);
+
 Cache *dcache, *icache;
 
 static int pow_of_two(int num)
@@ -89,6 +108,103 @@ static int pow_of_two(int num)
     return ret;
 }
 
+/*
+ * LRU evection policy: For each set, a generation counter is maintained
+ * alongside a priority array.
+ *
+ * On each set access, the generation counter is incremented.
+ *
+ * On a cache hit: The hit-block is assigned the current generation counter,
+ * indicating that it is the most recently used block.
+ *
+ * On a cache miss: The block with the least priority is searched and replaced
+ * with the newly-cached block, of which the priority is set to the current
+ * generation number.
+ */
+
+static void lru_priorities_init(Cache *cache)
+{
+    int i;
+
+    for (i = 0; i < cache->num_sets; i++) {
+        cache->sets[i].lru_priorities = g_new0(uint64_t, cache->assoc);
+        cache->sets[i].lru_gen_counter = 0;
+    }
+}
+
+static void lru_update_blk(Cache *cache, int set_idx, int blk_idx)
+{
+    CacheSet *set = &cache->sets[set_idx];
+    set->lru_priorities[blk_idx] = cache->sets[set_idx].lru_gen_counter;
+    set->lru_gen_counter++;
+}
+
+static int lru_get_lru_block(Cache *cache, int set_idx)
+{
+    int i, min_idx, min_priority;
+
+    min_priority = cache->sets[set_idx].lru_priorities[0];
+    min_idx = 0;
+
+    for (i = 1; i < cache->assoc; i++) {
+        if (cache->sets[set_idx].lru_priorities[i] < min_priority) {
+            min_priority = cache->sets[set_idx].lru_priorities[i];
+            min_idx = i;
+        }
+    }
+    return min_idx;
+}
+
+static void lru_priorities_destroy(Cache *cache)
+{
+    int i;
+
+    for (i = 0; i < cache->num_sets; i++) {
+        g_free(cache->sets[i].lru_priorities);
+    }
+}
+
+/*
+ * FIFO eviction policy: a FIFO queue is maintained for each CacheSet that
+ * stores accesses to the cache.
+ *
+ * On a compulsory miss: The block index is enqueued to the fifo_queue to
+ * indicate that it's the latest cached block.
+ *
+ * On a conflict miss: The first-in block is removed from the cache and the new
+ * block is put in its place and enqueued to the FIFO queue.
+ */
+
+static void fifo_init(Cache *cache)
+{
+    int i;
+
+    for (i = 0; i < cache->num_sets; i++) {
+        cache->sets[i].fifo_queue = g_queue_new();
+    }
+}
+
+static int fifo_get_first_block(Cache *cache, int set)
+{
+    GQueue *q = cache->sets[set].fifo_queue;
+    return GPOINTER_TO_INT(g_queue_pop_tail(q));
+}
+
+static void fifo_update_on_miss(Cache *cache, int set, int blk_idx)
+{
+    GQueue *q = cache->sets[set].fifo_queue;
+    g_queue_push_head(q, GINT_TO_POINTER(blk_idx));
+}
+
+static void fifo_destroy(Cache *cache)
+{
+    int i;
+
+    for (i = 0; i < cache->assoc; i++) {
+        g_queue_free(cache->sets[i].fifo_queue);
+    }
+}
+
 static inline uint64_t extract_tag(Cache *cache, uint64_t addr)
 {
     return addr & cache->tag_mask;
@@ -139,6 +255,11 @@ static Cache *cache_init(int blksize, int assoc, int cachesize)
     blk_mask = blksize - 1;
     cache->set_mask = ((cache->num_sets - 1) << cache->blksize_shift);
     cache->tag_mask = ~(cache->set_mask | blk_mask);
+
+    if (metadata_init) {
+        metadata_init(cache);
+    }
+
     return cache;
 }
 
@@ -155,12 +276,21 @@ static int get_invalid_block(Cache *cache, uint64_t set)
     return -1;
 }
 
-static int get_replaced_block(Cache *cache)
+static int get_replaced_block(Cache *cache, int set)
 {
-    return g_rand_int_range(rng, 0, cache->assoc);
+    switch (policy) {
+    case RAND:
+        return g_rand_int_range(rng, 0, cache->assoc);
+    case LRU:
+        return lru_get_lru_block(cache, set);
+    case FIFO:
+        return fifo_get_first_block(cache, set);
+    default:
+        g_assert_not_reached();
+    }
 }
 
-static bool in_cache(Cache *cache, uint64_t addr)
+static int in_cache(Cache *cache, uint64_t addr)
 {
     int i;
     uint64_t tag, set;
@@ -171,11 +301,11 @@ static bool in_cache(Cache *cache, uint64_t addr)
     for (i = 0; i < cache->assoc; i++) {
         if (cache->sets[set].blocks[i].tag == tag &&
                 cache->sets[set].blocks[i].valid) {
-            return true;
+            return i;
         }
     }
 
-    return false;
+    return -1;
 }
 
 /**
@@ -188,20 +318,28 @@ static bool in_cache(Cache *cache, uint64_t addr)
  */
 static bool access_cache(Cache *cache, uint64_t addr)
 {
+    int hit_blk, replaced_blk;
     uint64_t tag, set;
-    int replaced_blk;
-
-    if (in_cache(cache, addr)) {
-        return true;
-    }
 
     tag = extract_tag(cache, addr);
     set = extract_set(cache, addr);
 
+    hit_blk = in_cache(cache, addr);
+    if (hit_blk != -1) {
+        if (update_hit) {
+            update_hit(cache, set, hit_blk);
+        }
+        return true;
+    }
+
     replaced_blk = get_invalid_block(cache, set);
 
     if (replaced_blk == -1) {
-        replaced_blk = get_replaced_block(cache);
+        replaced_blk = get_replaced_block(cache, set);
+    }
+
+    if (update_miss) {
+        update_miss(cache, set, replaced_blk);
     }
 
     cache->sets[set].blocks[replaced_blk].tag = tag;
@@ -308,6 +446,10 @@ static void cache_free(Cache *cache)
         g_free(cache->sets[i].blocks);
     }
 
+    if (metadata_destroy) {
+        metadata_destroy(cache);
+    }
+
     g_free(cache->sets);
     g_free(cache);
 }
@@ -395,6 +537,28 @@ static void plugin_exit(qemu_plugin_id_t id, void *p)
     g_hash_table_destroy(miss_ht);
 }
 
+static void policy_init()
+{
+    switch (policy) {
+    case LRU:
+        update_hit = lru_update_blk;
+        update_miss = lru_update_blk;
+        metadata_init = lru_priorities_init;
+        metadata_destroy = lru_priorities_destroy;
+        break;
+    case FIFO:
+        update_miss = fifo_update_on_miss;
+        metadata_init = fifo_init;
+        metadata_destroy = fifo_destroy;
+        break;
+    case RAND:
+        rng = g_rand_new();
+        break;
+    default:
+        g_assert_not_reached();
+    }
+}
+
 QEMU_PLUGIN_EXPORT
 int qemu_plugin_install(qemu_plugin_id_t id, const qemu_info_t *info,
                         int argc, char **argv)
@@ -414,6 +578,7 @@ int qemu_plugin_install(qemu_plugin_id_t id, const qemu_info_t *info,
     iblksize = 64;
     icachesize = iblksize * iassoc * 32;
 
+    policy = LRU;
 
     for (i = 0; i < argc; i++) {
         char *opt = argv[i];
@@ -431,12 +596,26 @@ int qemu_plugin_install(qemu_plugin_id_t id, const qemu_info_t *info,
             dcachesize = g_ascii_strtoll(opt + 11, NULL, 10);
         } else if (g_str_has_prefix(opt, "limit=")) {
             limit = g_ascii_strtoll(opt + 6, NULL, 10);
+        } else if (g_str_has_prefix(opt, "evict=")) {
+            gchar *p = opt + 6;
+            if (g_strcmp0(p, "rand") == 0) {
+                policy = RAND;
+            } else if (g_strcmp0(p, "lru") == 0) {
+                policy = LRU;
+            } else if (g_strcmp0(p, "fifo") == 0) {
+                policy = FIFO;
+            } else {
+                fprintf(stderr, "invalid eviction policy: %s\n", opt);
+                return -1;
+            }
         } else {
             fprintf(stderr, "option parsing failed: %s\n", opt);
             return -1;
         }
     }
 
+    policy_init();
+
     dcache = cache_init(dblksize, dassoc, dcachesize);
     if (!dcache) {
         const char *err = cache_config_error(dblksize, dassoc, dcachesize);
@@ -453,8 +632,6 @@ int qemu_plugin_install(qemu_plugin_id_t id, const qemu_info_t *info,
         return -1;
     }
 
-    rng = g_rand_new();
-
     qemu_plugin_register_vcpu_tb_trans_cb(id, vcpu_tb_trans);
     qemu_plugin_register_atexit_cb(id, plugin_exit, NULL);
 
-- 
2.25.1